This invention relates to optical mirrors and is particularly directed to fluid-cooled mirrors utilized in laser systems.
Metallic mirrors are commonly used to focus, deflect or change the shape of laser beams. One system for aiming a laser beam and focusing it on a target, described in U.S. Pat. No. 3,514,776 to Mulready, includes a concave mirror having an adjustable reflecting concave face which is capable of being aimed at the target. The curvature of the reflecting surface of the mirror can be adjusted by pressure or mechanical means. Similar systems such as the ones disclosed in U.S. Pat. No. 2,555,387 to Zobel and U.S. Pat. No. 3,967,899 to O'Meara additionally utilize magnetic or electrical actuators, respectively, to modify mirror face plate geometry. However, none of the prior patents mentioned show any means for compensating for thermal distortions. As is evident to one of ordinary skill in the art, these thermal distortions are a result of the stresses induced in the structural material of the mirror as it absorbs some of the radiant energy of an impinging laser beam. Thus the actuators modifying face plate geometry must not only be able to supply prerequisite mechanical forces to manipulate the curvature of a mirror's front face but must also provide sufficient mechanical forces to compensate for the induced thermal stresses. Thus the design of the actuators tend to be complicated and expensive.
Typically, the mirrors which are subjected to intense heat by the laser beam require cooling of the optically reflective surface of the mirror since only minimal thermal distortion thereof due to differential temperatures can be permitted. Examples of liquid cooled mirrors include U.S. Pat. No. 3,637,296 to McLafferty et al. and U.S. Pat. No. 3,731,992 to Mansell. However a disadvantage of these mirror structures lies in the location of the inlet and outlet manifolds which allow uneven heat exchange to exist over the mirror surface. Such differential cooling could cause slight distortion of the mirror surface and thus undesirable distortion of the reflected laser beam. Other methods such as shown in U.S. Pat. No. 3,923,383 to Engel et al, U.S. Pat. No. 3,884,558 to Dunn, III et al, and U.S. Pat. No. 3,708,223 to Sorensen et al. use complicated manifolding designs in attempting to achieve uniform heat exchange. However, inasmuch as these mirrors are cooled parallel to their back surfaces hot spots on the mirror can not be effectively obviated. Hence, both axial and radial temperature gradients still exist creating high thermal stresses. Also, mirror deflections are not controlled by face plate geometry adjustments so that optimum laser beam reflection is unobtainable with the above referenced patents. Large pressure is required to force the coolant through its channels resulting in large forces acting on the mirror. Thus, the above systems are structurally heavy inasmuch as the mirror systems must resist the high pressure coolants. Finally, the strength requirements imposed by the high pressure cooling systems and the complexity of the manifold designs result in high manufacturing costs.